The invention relates to active acoustic attenuation systems, and provides a system for limiting output power of the correction signal to the canceling output transducer.
The invention arose during continuing development efforts relating to the subject matter shown and described in U.S. Pat. Nos. 4,677,676, 4,677,677, 4,736,431, 4,815,139, 4,837,834, 4,987,598, 5,022,082, and 5,033,082, incorporated herein by reference.
Active attenuation involves injecting a canceling acoustic wave to destructively interfere with and cancel an input acoustic wave. In an active acoustic attenuation system, the output acoustic wave is sensed with an error transducer such as a microphone which supplies an error signal to a control model which in turn supplies a correction signal to a canceling output transducer such as a loudspeaker which injects an acoustic wave to destructively interfere with and cancel the input acoustic wave. The acoustic system is modeled with an adaptive filter model.
In some applications, the acoustic pressure level of the input acoustic wave may exceed the ability of the canceling output transducer to cancel same. An example is a sudden change in the input noise level, for instance sudden engine acceleration in automotive exhaust silencing applications. During this condition, the active noise controller may become unstable if it is allowed to adapt and output a correction signal which is beyond the capability of the canceling loudspeaker or otherwise attempt to overdrive same. When the input noise decreases to normal levels, e.g. upon termination of the sudden acceleration, the control model will have to re-adapt and converge new weight update coefficients.
In one aspect of the present invention, overdriving of the canceling output transducer is prevented by engaging a power limiting function which is accomplished by shunting at least part of the correction signal to a shunt path and away from the output transducer. The shunt path is in parallel with the output transducer and when engaged at high input noise levels enables the adaptive filter model to remain stable and converged, with part of the correction signal still going to the canceling output transducer and the remainder of the correction signal going through the shunt path around the output transducer, while the adaptive filter model continues to adapt.
In another aspect, variable gains are provided in one or both of the shunt path and the input to the output transducer. The ratio between the part of the correction signal supplied to the output transducer and the part of the correction signal shunted to the shunt path is varied.
In another aspect, a second adaptive filter model is provided and models the output transducer and the error path, and the shunt path is provided through a copy of such second model.
In another aspect, the power limiter is engaged when the part of the correction signal supplied to the output transducer exceeds an engagement threshold, and is disengaged when a calculated correction signal, theoretically needed for full cancellation, decreases below a disengagement threshold. If the part of the correction signal supplied to the output transducer is greater than a given range, then the part of the correction signal supplied to the output transducer is decreased and the part of the correction signal shunted to the shunt path is increased. If the theoretically needed correction signal is less than another given range, then the part of the correction signal supplied to the output transducer is increased and the part of the correction signal shunted to the shunt path is decreased.